During the past two decades much effort has been expended on developing and disseminating standardised approaches to CTG interpretation. Here in Australia, the Royal Australian and New Zealand College of Obstetricians and Gynaecologists (RANZCOG) has generated the Intrapartum Fetal Surveillance Guideline, coupled with the Fetal Surveillance Education Progam. In other parts of the world the International Federation of Obstetrics and Gynaecology (FIGO) or the National Institute for Child Health and Human Development (NICHHD) guidelines, or others, are used.
While each guideline is slightly different, they share a common belief that the same fetal heart rate pattern has the same meaning in different fetuses. Is this actually the case?
Fetal sex
There is evidence of differences in fetal heart rate patterns in different groups of fetuses. First, fetal sex makes a difference (Porter et al., 2016). Healthy male fetuses are more likely than healthy female fetuses to have late decelerations and repetitive variable decelerations in the half hour prior to birth. The total deceleration area in the CTG traces of healthy female fetuses was significantly less than that of healthy male fetuses. None of these fetuses was acidaemic or otherwise unwell at birth. It is therefore important to ask whether the indicators commonly used to make a recommendation for surgical birth are reliable for male fetuses.
Fetal age
The second way that fetal heart rate patterns change for different fetuses is a reduction in the baseline heart rate with increasing gestational age. It is therefore important to consider what the significance of a high but “normal” baseline heart rate is for fetuses who are older than 40 weeks of gestational age. Guidelines typically present a baseline heart rate of 110 – 160 beats per minute as “normal”.
This is the question Ghi et al. (2020) asked recently. They retrospectively reviewed outcomes for fetuses of at least 40 weeks of gestation during the first hour of active spontaneous labour. Fetuses with a baseline heart rate of 150 – 160 beats per minute were more likely to be acidotic at birth, have meconium stained liquor, or be born by caesarean section than fetuses with a baseline heart rate of 110 – 149 baseline heart rate. It appears that for fetuses over 40 weeks of gestation, using the standard definition of 110 – 160 beats per minute for the baseline heart rate may falsely reassure clinicians of fetal wellbeing for a subset of fetuses.
Individualised interpretation
These two examples demonstrate the challenges of applying a single standard to a varying population who have individual physiological characteristics. For intrapartum fetal heart rate monitoring to live up to expectation we need more evidence and the ability to apply that evidence to the individual woman and her fetus. A one-size-fits all application of intrapartum fetal heart rate interpretation guidelines may serve a proportion of the population being monitored, but will always leave a significant minority at a disadvantage.
References
Ghi, T., Di Pasquo, E., Dall’asta, A., Commare, A., Melandri, E., Casciaro, A., Fieni, S., & Frusca, T. (2020, Oct 13). Intrapartum fetal heart rate between 150 and 160 bpm at or after 40 weeks and labor outcome. Acta Obstetrica Gynecologica Scandanavica, in press. https://doi.org/10.1111/aogs.14024
Porter, A. C., Triebwasser, J. E., Tuuli, M. G., Caughey, A. B., Macones, G. A., & Cahill, A. G. (2016, Jul). Fetal Sex Differences in Intrapartum Electronic Fetal Monitoring. American Journal of Perinatology, 33(8), 786-790. https://doi.org/10.1055/s-0036-1572531
Tags: fetal age, fetal sex, guidelines
Birth Small Talk 
Excellent and thought provoking post as usual Kirsten. Thank you. Great questions and the answer to are all fetuses the same? Absolutely not. They are as unique as their mothers.
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Now I must go back through my records to see if can pull this out. And I have modified my charts to document going forward.
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